1. Field of the Invention
The present invention relates to a multiplier and a radio communication apparatus using the multiplier.
2. Description of the Related Art
In the radio communication field as of 2005, there are adopted many systems for modulating a signal to be transmitted with respect to both the amplitude and phase of the signal. Therefore, in general, a quadrature modulator is used in a transmitter, and a quadrature demodulator is used in a receiver.
In a general quadrature modulator, as described in, for example, Jpn. Pat. No. 3360912, two multipliers multiply two modulating signals called an in-phase signal (referred to as the I-signal) and a quadrature-phase signal (referred to as the Q-signal) and two local signals having a phase difference of 90° from each other. Multiplied output signals from the multipliers are combined, and a modulated signal is generated. Assuming that the I-signal is I(t), the Q-signal is Q(t), and two local signals are cos ωt and Sin ωt, the modulated signal output from the quadrature modulator is represented by the following formula:I(t)cos ωt+Q(t)sin ωt  (1)
On the other hand, the quadrature demodulator performs processing reverse to that of the quadrature modulator. That is, in a general quadrature demodulator, as described in, for example, Jpn. Pat. No. 3545615, two multipliers multiply the modulated signal represented by Formula (1) by two local signals having a phase difference of 90° from each other. As a result, a multiplied output signal represented by the following equation is obtained from each multiplier:
                                                        cos              ⁢                                                          ⁢              ω              ⁢                                                          ⁢                              t                ⁡                                  (                                                                                    I                        ⁡                                                  (                          t                          )                                                                    ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                        +                                                                  Q                        ⁡                                                  (                          t                          )                                                                    ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                                        )                                                      =                                                            I                  ⁡                                      (                    t                    )                                                  ⁢                                                      1                    +                                          cos                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                                        2                                            +                                                Q                  ⁡                                      (                    t                    )                                                  ⁢                                                      sin                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    ω                    ⁢                                                                                  ⁢                    t                                    2                                                              ;                ⁢                                  ⁢        and                            (        2        )                                          sin          ⁢                                          ⁢          ω          ⁢                                          ⁢                      t            ⁡                          (                                                                    I                    ⁡                                          (                      t                      )                                                        ⁢                                                                          ⁢                  cos                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                  t                                +                                                      Q                    ⁡                                          (                      t                      )                                                        ⁢                                                                          ⁢                  sin                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                  t                                            )                                      =                                            I              ⁡                              (                t                )                                      ⁢                                          sin                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                t                            2                                +                                    Q              ⁡                              (                t                )                                      ⁢                                          1                -                                  cos                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                  t                                            2                                                          (        3        )            
After passing the multiplied output signal of each multiplier through a low-pass filter to remove a high-frequency component, the signal is amplified with an appropriate gain to obtain an original I-signal I(t) and Q-signal Q(t).
In actual quadrature modulator and quadrature demodulator, a double balanced mixer is generally used in the multiplier so as to reduce undesired signals included in an output. In this case, in a baseband signal, a differential type is used in which a positive signal and a negative signal are combined.
In the conventional quadrature modulator and quadrature demodulator, differential circuits corresponding to the I-signal and the Q-signal need to be used, respectively, in order to avoid an output of an undesired signal. This increases the number of analog elements such as transistors, resistors, and capacitors required for the quadrature modulator and the quadrature demodulator. As a result, in a case where a radio communication apparatus is integrated on a semiconductor circuit, an area occupied by the quadrature modulator and the quadrature demodulator on a semiconductor chip increases. With the progress of a semiconductor fine working technology, a cost of the semiconductor chip per unit area is on the rise. In consideration of this, it is an important issue to reduce occupying areas of individual functional elements such as the quadrature modulator and the quadrature demodulator.
Moreover, since the multipliers corresponding to the I-signal and the Q-signal are used in the conventional quadrature modulator and quadrature demodulator, an I-signal gain does not necessarily meet a Q-signal gain. Therefore, a modulation error or a demodulation error is generated causing a communication quality to deteriorate.